(1) Field of the Invention
The invention relates to centrifugal blowers and is directed more particularly to a blower having means for reducing the operational noise thereof.
(2) Description of the Prior Art
Centrifugal blowers, also known as scroll-type blowers, sirocco blowers, etc., are well known in the art and include a generally cylindrically shaped impeller or blower wheel, having a plurality of blades, the impeller being rotatably mounted within a housing. The blades are oriented in the impeller such that as the impeller is rotated in a specific direction, air is drawn axially into the interior of the impeller and blown radially outwardly by its blades, and then through a tangential output at increased pressure. The tangential outlet is formed in the housing of the blower and includes a sharply radiused edge or cutoff. The cutoff edge is typically positioned closer to the impeller than any other part of the blower housing and defines a sharp separation between the output and the interior of the blower housing. As a result of the impeller rotating at relatively high speeds past the cutoff, an interaction is created therebetween which produces a pure tone, or whistling noise, at a fundamental frequency. This fundamental frequency has been empirically determined as the speed of the blower wheel in RPM divided by 60, multiplied by the number of blades contained in the impeller. In a typical impeller consisting of nine blades operated at 3,433 RPM, the fundamental blade pass frequency, or pure tone frequency, would be 515 Hz. In addition to the fundamental frequency, this particular design would also have a second harmonic tone at 1030 Hz, a third harmonic at 1545 Hz, etc., as is well known in the art. These pure tones represent frequencies of dramatically increased amplitude over the average or "white" noise ordinarily produced by the motor and blower in connection with its rotating action. Also, as is well known, these pure tones of increased amplitude are produced at each of the harmonics, including the third and fourth harmonic, which can generate tones which are particularly annoying to a human ear. For example, a fourth harmonic of the example noted above occurs at approximately 2,060 Hz, which is an ear-piercing sound particularly bothersome at these increased amplitudes. Therefore, there typically is significant design activity and effort expended to eliminate these pure tones. However, the inventors herein are not aware of any ready solution to this problem, until now.